Molecular Characteristics of Catalytic Nitrogen Removal from Coal Tar Pitch over γ-Alumina-Supported NiMo and CoMo Catalysts

The removal of nitrogen from coal tar pitch (CTP) through the hydrodenitrogenation (HDN) of CTP and its molecular behavior were evaluated in the presence of NiMo/γ-alumina and CoMo/γ-alumina catalysts. Fourier transform ion cyclotron resonance mass spectrometry with atmospheric pressure photoionization was used to analyze the complicated chemical classes and species of CTP and the treated products at the molecular level. Nitrogen species were qualitatively analyzed before and after hydrotreatment. A single-stage hydrotreatment with an HDN catalyst resulted in a high sulfur removal performance (85.6–94.7%) but a low nitrogen removal performance (26.8–29.2%). Based on relative abundance analyses of nitrogen and binary nitrogen species, CcHh-NnSs was the most challenging species to remove during HDN treatment. Furthermore, prior hydrodesulfurization was combined with HDN treatment, and the dual hydrotreatments yielded a significantly improved nitrogen removal performance (46.4–48.7%).


Introduction
Needle coke is produced by heating petroleum or coal tar pitch (CTP) to high temperatures, causing the material to form elongated needlelike structures [1].
Needle coke produced using petroleum pitch is typically of a higher quality than that produced using CTP because of the higher purity and lower nitrogen content of petroleum pitch [2], which is a by-product of petroleum refining. Petroleum pitch is obtained from the residue of crude oil distillation, whereas CTP, which is a by-product of coal processing, is obtained by distilling coal tar (CT) [3].
The selection of needle coke produced using petroleum pitch or CTP depends on several criteria, including the quality, purity, availability, and cost of the pitch and the desired properties of the needle coke. Therefore, CTP remains of interest because of its low cost and commercial-scale availability [4].
CT, as a source of CTP, is a by-product of coal processing and contains various contaminants, including nitrogen-containing compounds such as pyridine, quinoline, and indole [5]. The removal of nitrogen from CT or CTP is essential in reducing their environmental effects and preventing puffing in various industrial applications. Table 1 lists the measured sulfur and nitrogen contents in the feed and hydrotreated products. The net changes in the total amounts of sulfur and nitrogen in the CTP samples during hydrotreatment were calculated, and the results are summarized in Table 1.
The C/H ratio in the hydrotreated CTP decreases gradually during the single and dual stages of operation over the NMA and CMA catalysts. The total change in the C/H ratio during the single-stage reaction is 15.9% over NMA and 14.3% over CMA, indicating that hydrotreatment over CMA exhibits a higher activity than that over NMA. The dual-stage reaction displays a different trend (both catalysts: 14.8). The total sulfur content in CTP also gradually decreases during the single and dual stages of operation over NMA and CMA. However, the conversion (94.7 wt.%) of the total sulfur content in CTP over CMA is significantly higher than that (86.5 wt.%) over NMA, suggesting that the sulfur species are more easily hydrotreated over CMA. Moreover, an ideal removal of sulfur species is observed during the dual-stage reaction, which is attributed to the high selectivity toward sulfur over the second-stage catalysts, after the diluted catalyst (DM5CQ) is used in the first-stage reaction. Significantly, the total nitrogen content in CTP does not decrease sufficiently in each stage. Even the net amount of nitrogen in CTP decreases by up to 40.0% compared to the original amount of nitrogen in the feed. This is because the nitrogen species in CTP are located at the sterically hindered sites of the carbon agglomerates and/or polar nitrogen species occupying the acid/active sites of hydrogenolysis over the catalysts.

Reactivities of the Molecular Classes
A comparison of the results of bulk analysis and FT-ICR MS remains controversial, as shown in the study by Rodgers [25], wherein the results of FT-ICR MS are explained based on the C/H ratios obtained via bulk analysis. Even when several constraints are assumed owing to the preferential ionization (APPI) of highly aromatic/condensed CTP and its heterogeneous aggregation properties, comparing the fractions of CTP in detail in terms of their polarities and C/H molar ratios should be insightful. Notably, FT-ICR MS with APPI is limited in reflecting the complete, precise molecular distribution, as its selective ionization effectiveness for different molecules may vary. However, the selective ionization effectiveness for the same molecules or class of molecules should be identical or similar. This enables the use of the FT-ICR MS spectra to interpret the relative changes in the various classes of molecules in CTP during hydrotreatment. Tables 2 and 3 list the relative abundance of each molecular class in CTP after the single-and dual-stage reactions, respectively, as the weighted average intensities due to radical actions and protonated species generated by the positive APPI. The net change in each molecular class during hydrotreatment was calculated based on the difference in each molecular class of the feed and products, and the results are also presented in Tables 2 and 3. After the single-stage reaction, the relative abundances of most classes containing sulfur (C c H h -S s , C c H h -O o S s , and C c H h -N n O o S s ) decrease, whereas the relative abundance of the binary-coordinated sulfur-nitrogen class (C c H h -N n S s ) increases. Notably, the relative abundance of the ternarycoordinated sulfur-nitrogen-oxygen class decreases over NMA and CMA during the single-stage reaction. The increase in the amount of C c H h may be attributed to an increase in heteroatom-free hydrocarbons due to effective hydrogenolysis. Therefore, the classes with binary-coordinated sulfur-nitrogen heteroatoms are located at the sterically hindered sites, less active than the single-/ternary-coordinated sulfur classes, and challenging to remove from CTP during hydrotreatment. A comparison of the heteroatom classes reveals that the unitary heteroatom classes in CTP are removed in the order of C c H h -S s , C c H h -O o , and C c H h -N n . The binary heteroatom classes in the asphaltenes are removed in the order Additionally, the net amount of the C c H h -N n S s class in the first step of the dual-stage reaction increases slightly, by approximately 10%. The increase in the C c H h -N n S s class in the products during the single-or first-stage reaction may be attributed to (1) the significantly lower HDN reactivities of the classes with nitrogen heteroatoms; (2) the conversion of C c H h -N n O o S s via HDO to C c H h -N n S s , as the reactivities of heteroatom species generally decrease in the order HDS > HDO > HDN; and (3) the polycondensation of the C c H h -N n S s of CTP. Table 3 also indicates the increase in the net amount of C c H h in the products after the single-and dual-stage reactions, which may be attributed to the removal of the heteroatoms of the heteroatom-containing classes in CTP. A detailed discussion on the analysis of the data obtained via (APPI) FT-ICR MS is provided in the subsequent section to analyze the changes in the abundances of various molecular classes and their subclasses, with the DBE values and carbon number distributions.

C c H h Class
The behavior of the C c H h class was characterized via the distribution of the DBE values, as shown in Figure 1. The DBE values of the feed and products (NMA and CMA) are in the range 5-40. Even when the distribution of the DBE values is bimodal (A and B ranges), the B-range abundance (%) is reduced after HDN in the single-stage reaction. However, each HDN (second-stage) after the first-stage reaction in the dual-stage reaction leads to an increased B-range abundance. This may be explained as follows: (1) the hydrogenolysis of the species in the B range in the single-stage reaction is dominant, and (2) in the dual-stage reaction, the first-stage catalyst (DM5CQ) may catalyze the hydrogenation of the species in the entire DBE range, with a relatively homogeneous hydrocracking of the hydrogenated molecules over the second-stage catalyst. The C/H ratio after the dual-stage reaction is slightly reduced compared to that after the single-stage reaction. However, the DBE values in all ranges are relatively similar for the feed and products, and the carbon yield of coking should be maintained.
genolysis of the species in the B range in the single-stage reaction is dominant, and (2) in the dual-stage reaction, the first-stage catalyst (DM5CQ) may catalyze the hydrogenation of the species in the entire DBE range, with a relatively homogeneous hydrocracking of the hydrogenated molecules over the second-stage catalyst. The C/H ratio after the dualstage reaction is slightly reduced compared to that after the single-stage reaction. However, the DBE values in all ranges are relatively similar for the feed and products, and the carbon yield of coking should be maintained.    Table 4 summarizes the relative abundances of the unitary, binary, and ternary nitrogen heteroatoms in CTP during each stage of CTP hydrotreatment. The sulfur species in CTP may be easily removed, indicating the presence of highly reactive species, whereas  For the single-stage reaction, the values of the L slopes are slightly decreased, which is attributed to the hydrogenation effect, whereas the H slopes after the reaction are higher than that of the feed, which may be attributed to the sites of the removed heteroatoms of the agglomerates of the C c H h heteroatoms.

Subclasses of Nitrogen
the dual-stage reaction, the first-stage catalyst (DM5CQ) may catalyze the hydrogenation of the species in the entire DBE range, with a relatively homogeneous hydrocracking of the hydrogenated molecules over the second-stage catalyst. The C/H ratio after the dualstage reaction is slightly reduced compared to that after the single-stage reaction. However, the DBE values in all ranges are relatively similar for the feed and products, and the carbon yield of coking should be maintained.    Table 4 summarizes the relative abundances of the unitary, binary, and ternary nitrogen heteroatoms in CTP during each stage of CTP hydrotreatment. The sulfur species in CTP may be easily removed, indicating the presence of highly reactive species, whereas  Table 4 summarizes the relative abundances of the unitary, binary, and ternary nitrogen heteroatoms in CTP during each stage of CTP hydrotreatment. The sulfur species in CTP may be easily removed, indicating the presence of highly reactive species, whereas the removal of nitrogen species is challenging, which is attributed to the refractory nitrogen species with steric hindrance. Therefore, a detailed investigation of the molecular behavior in nitrogen removal is required to characterize this process. Unitary species are the most abundant in the feed, followed by the binary and then ternary nitrogen species. The total abundance of C c H h -N n decreases during the single-and dual-stage reactions. However, the abundance of C c H h -N 4 species increases, regardless of the reaction type, possibly because of the partial condensation of N 1 , N 2 , or N 3 . The C c H h -N n compounds mainly contain N 1 species, and these species appear to lead to HDN during the single-and dual-stage reactions. The binary nitrogen species C c H h -N 1 S 3 , -N 1 S 4 , -N 2 S 2 , and -N 3 S 1 are refractory, whereas C c H h -N 1 S 1 represents a relatively reactive species. The more favorable HDO of CTP compared to HDN effectively removes the C c H h -N n O o species. Research on the development of an effective process for upgrading CTP to high-quality carbon materials should focus on the design of novel catalysts with higher HDN activities. The distribution of the DBE values follows the C c H h trend, as shown in Figure 3. However, the DBE values are significantly reduced in the single-and dual-stage reactions, indicating the effective removal of single heteroatoms, such as the C c H h -N n species. The second-stage reaction, in particular, results in a significant reduction in the amount of C c H h -N n species after mild catalytic hydrogenation in the first stage. However, whether the products of the C c H h -N n species after the second-stage reaction occur in sterically hindered forms is unknown. This may be confirmed by the introduction of a hybrid process, such as catalysis and adsorption. Figure 4 shows the DBE values of C c H h -N n depending on the carbon number. Single slopes are obtained for the feed and products of the single-and dual-stage reactions, in contrast to those of the C c H h species. The slopes obtained for all products are low, and nitrogen species are removed via ring opening after hydrogenation. The slopes decrease significantly (0.69 → 0.65) during the dual-stage reaction, indicating accelerated HDN after the first-stage reaction.    Table 5 summarizes the characteristics of the CTP used in this study. Single-and dual-stage reactions were performed. The commercial catalysts (NMA and CMA) were used as received from JGC Catalysts and Chemicals (JGC C&C, Kawasaki, Japan) in the single-stage reaction. To address the active sites [26,27], the active sites over NiMo and CoMo can be elucidated to symmetric stretching of the Mo-O bond in bridged or twodimensional polymeric forms of octahedrally coordinated Mo oxide species. Such Mo oxide species have been shown to interact weakly with supports, resulting in higher reducibility and activity in hydrotreating reactions [26]. Moreover, In the Ni(Co)-Mo-S model, Ni(or Co)S is considered the source of promoter atoms and is located in any of the fivefold coordinated sites at the (1010) edge planes of MoS2. The distribution of MoS slabs over support materials is closely related to the presence of promoters such as Ni and Co, although the sulfidation of calcined catalysts may lead to the redistribution of surface species. The lower number of layered stacks (MoS) leads to a shorter distance between the active sites and the support materials, thereby resulting in improved HDS via effective     Table 5 summarizes the characteristics of the CTP used in this study. Single-and dual-stage reactions were performed. The commercial catalysts (NMA and CMA) were used as received from JGC Catalysts and Chemicals (JGC C&C, Kawasaki, Japan) in the single-stage reaction. To address the active sites [26,27], the active sites over NiMo and CoMo can be elucidated to symmetric stretching of the Mo-O bond in bridged or twodimensional polymeric forms of octahedrally coordinated Mo oxide species. Such Mo oxide species have been shown to interact weakly with supports, resulting in higher reducibility and activity in hydrotreating reactions [26]. Moreover, In the Ni(Co)-Mo-S model, Ni(or Co)S is considered the source of promoter atoms and is located in any of the fivefold coordinated sites at the (1010) edge planes of MoS2. The distribution of MoS slabs over support materials is closely related to the presence of promoters such as Ni and Co, although the sulfidation of calcined catalysts may lead to the redistribution of surface species. The lower number of layered stacks (MoS) leads to a shorter distance between the active sites and the support materials, thereby resulting in improved HDS via effective  Table 5 summarizes the characteristics of the CTP used in this study. Single-and dualstage reactions were performed. The commercial catalysts (NMA and CMA) were used as received from JGC Catalysts and Chemicals (JGC C&C, Kawasaki, Japan) in the single-stage reaction. To address the active sites [26,27], the active sites over NiMo and CoMo can be elucidated to symmetric stretching of the Mo-O bond in bridged or two-dimensional polymeric forms of octahedrally coordinated Mo oxide species. Such Mo oxide species have been shown to interact weakly with supports, resulting in higher reducibility and activity in hydrotreating reactions [26]. Moreover, In the Ni(Co)-Mo-S model, Ni(or Co)S is considered the source of promoter atoms and is located in any of the fivefold coordinated sites at the (1010) edge planes of MoS 2 . The distribution of MoS slabs over support materials is closely related to the presence of promoters such as Ni and Co, although the sulfidation of calcined catalysts may lead to the redistribution of surface species. The lower number of layered stacks (MoS) leads to a shorter distance between the active sites and the support materials, thereby resulting in improved HDS via effective hydrogenolysis [27]. As-received NMA and CMA catalysts have been characterized by XRF (ZSX Primus IV, Tokyo, Japan), N 2 -sorption isotherm (TriStar II3020, Norcross, GA, USA), TEM (HITACHI HF5000, Tokyo, Japan), Raman (LabRAM HR-800, Tokyo, Japan), and NH 3 -TPD (BELL CAT II, Osaka, Japan) and are presented in the Supplementary Information together with chemical stability data. The dual-stage reaction was performed using a MoO 3 /γ-alumina catalyst (DM5CQ, JGC C&C) in the first reaction to conduct hydrogenation and partial hydrodesulfurization, and NMA and CMA were then introduced in the following (second) reaction to complete the dual reaction. Table 6 lists the characteristics of the basic catalysts.

Hydrotreatment Apparatus
The catalysts were sulfidized prior to hydrotreatment using a gaseous mixture containing 5 vol.% H 2 S in flowing hydrogen at 360 • C for 2 h. CTP hydrotreatment was conducted in a 150 mL autoclave reactor equipped with a sampling port, and the reaction was conducted in one or two stage(s). During a typical experiment, 30 g of feed was hydrotreated in the presence of 3 g of a sulfidized catalyst at 350 • C under 6 MPa hydrogen atmosphere (initial pressure at room temperature). The heating time required to reach the reaction temperature was 0.5 h, and the agitation speed was maintained at 400 rpm to avoid external mass transfer limitations. The reaction time for the one-stage operation was 6 h after reaching the target temperature. In the two-stage operation, first-stage hydrodesulfurization (HDS) was performed for 6 h. This was followed by rapid cooling of the reactor to room temperature and a subsequent second-stage operation with fresh hydrogen. The two-stage operation, wherein the gaseous product was replaced with fresh hydrogen, was conducted to effectively remove heteroatoms over the catalyst.

Bulk Analysiss
The products of the hydrogenation of CTP were used without pretreatment, and an elemental analyzer (EA1110, CE Instruments, Wigan, UK) was used to determine the bulk sulfur and nitrogen contents in CTP.

FT-ICR MS
The 7 T FT-ICR M spectrometer (SolariX 2XR, Bruker, Billerica, MA, USA) used in this study was equipped with a detection system that enabled detection at twice the cyclotron frequency [28]. Nitrogen was used as the drying and nebulization gas; argon was supplied to the collision cell; and the samples were directly injected using a syringe pump (Harvard Apparatus, Holliston, MA, USA). The stock solutions were prepared by diluting the samples in toluene to approximately 0.5 mg/mL, and the APPI source (APPI II, Bruker Billerica, MA, USA) was used in the positive mode. The sample was charged at a flow rate of 300 µL/h. The drying gas temperature and flow rate were maintained at 220 • C and 3.8 L/min, respectively, and the pressure of the nebulizer gas was set to 3.3 bar. The capillary voltage was 3.5 kV. The source was heated to 430 • C, and the mass spectra were acquired between m/z 180 and 1200. The dataset size was set to 8 M words, and the ion accumulation time was 7.2 s. A total of 200 data points were summed to generate the